Immediately,in the host of Metelluswhich was the beginning of the conspiracy,Scipio gathered up;the plan of the youths was brought forward by those (de quibus?),which he had found,drawn by the sword above the heads of the conspirators

se contulit: the reflexive pronoun here is important; here is what Lewis & Short says about the reflexive use of the verb - "conferre se, to betake or turn one's self anywhere, to go"

cum concilium invenisset: you want the first meaning of concilium here in the dictionary: "a collection of people, an association, gathering, union, meeting, assembly" (this is not consilium, but concilium)

de quibus allatum erat - this is like a parenthetical phrase which you can actually take out of the sentence; it is just remarking on exactly which young men we are talking about here - "the assembly of the young men [[[about whom (de quibus) he had been informed (allatum erat)]]]"

stricto gladio: an ablative absolute in Latin does not have a grammatical relationship to the main subject (that is why it is called an "absolute"), but it often has a logical relationship, and if your goal is to translate into English, often you need to translate that logical relationship for it to make sense:When he found the gathering of young men, his sword having been drawn... (grammatical)When he found the gathering of young men, he drew his sword... (logical)

You will find out in the next sentence what he plans to do with that sword!

I think the latin here works grammatically only with ellipsis, so ellipsis must be considered self-evident (unless I'm mistaken), since an ablative absolute won't be a main clause, and the most obvious reason for an omission here is the use of the same verb "invenit" (or something very close). "When he found the gathering of young men, he drew his sword" is OK but I think it is more dramatic and more faithful to the Latin to say this: "Whenever* he [had] found the meeting of young men there, he came upon it with his sword drawn over the heads of those plotting."

Hi Adrian, I think the problem you are referring to is a matter of poor punctuation choice in the presentation of the text - the main verb of the "sentence" is really the following inquit, yes? I would have sent up the sentences like this, with inquit as the main verb of the final sentence:Statim in hospitium Metelli,(qui conspirationis erat princeps)se contulit Scipio.Cumque concilium ibi iuvenum(de quibus allatum erat)invenisset,stricto super capita consultantium gladio,"Iurate," inquit, "vosneque rempublicam populi Romani deserturos,neque alium civem Romanum deserere passuros;qui non iuraverit,in se hunc gladium strictum essesciat."

Well, a main verb had to come from somewhere to make it grammatical and ellipsis is an alternative as is your solution.Ut illa sententia grammaticè legatur, verbum princeps alicundè quidem venire debet et ellipsis alternatam explicationem secus tuam complet.

I also have a ‘literal translation’ by Edward Roth (Philadelphia, David McKay, 1898). For the sake of interest, I’ll give his version of the passage, framed by a bit of context.

After the disaster at Cannae (BC 216 when Hannibal thrashed the Roman army) young Scipio was given command of what soldiers remained. He heard that certain young noblemen were plotting to abandon Italy.

Here’s the passage in question:

Immediately proceeding to the house of Metellus, who was the head of the conspiracy, and there finding the young men in session concerning whom the information had been given, drawing his sword and holding it over their heads as they were deliberating, “Swear”, he cried, “swear that you will neither desert Rome yourselves nor allow any Roman citizen to do so!”

The text continues: “Whoever does not take this oath must be informed that it is against himself that this sword is drawn.” Just as much frightened as if they saw the victorious Hannibal himself, they all took the oath and surrendered themselves to Scipio to be kept in custody.